Seismic exploration is a process for assessing the characteristics of underground land formations by generating and sensing seismic waves. In general, an acoustic energy source generates seismic acoustic waves which travel through subterranean formations. The waves are reflected back to the surface by formation interfaces associated with different formation densities, and the waves may also be refracted to travel along the interface before returning to the surface. Seismic detonators and associated booster charges are placed in boreholes at or near the surface to provide a seismic wave source. Geophones or other acoustic energy transducers detect the acoustic waves reflected or refracted back to the surface, where an array of such transducers is typically positioned at the surface for land surveys with individual transducers spaced from one another at known intervals and distances from the seismic source. Data from the transducers can be correlated in time with the time at which the seismic source was actuated (detonated), and analyzed to provide information regarding the structure of the earth underneath the transducers, for example, in oil and gas exploration.
The time when the seismic source detonator fires is known as the “time break”, and it is desirable to confirm actual detonation of a seismic charge by providing a value known as a confirmation time break (CTB) indicating an actual time at which a seismic detonator was successfully fired, either directly as a real-time value or as a time difference between the actual firing and the time at which the firing command or signal was issued. In particular, certain applications involve a large number (e.g. thousands) of acoustic sensors or transducers connected to data acquisition systems for obtaining acoustic sensor data, and operation of the sensors and data acquisition system is expensive. Thus, enabling transducers and acquiring corresponding data is costly if a seismic detonator does not actually detonate an associated booster charge. Consequently, confirmation of actual successful seismic source detonation by way of a confirmation time break signal or value is highly desired to signal the seismic data acquisition to enable the array of transducers in the field.
In the past, the time break was usually confirmed by detecting a current surge into an electric detonator (e.g., current interruption as described in U.S. Pat. No. 3,851,589 and monitoring a derivative of current change described in U.S. Pat. No. 2,331,627), or a fixed predetermined delay after the transmission of a firing command to an electronic detonator. However, the seismic charge actually explodes some period of time after the firing signal or command, and the time varies. As a result, the reported time break does not represent the actual time of detonation, leading to inaccuracy in temporal correlation of the acoustic sensor data. Moreover, the provision of a firing signal or command does not ensure that the detonator or the seismic charge will go off, particularly if there is excessive leakage or shorts in the bus wires for electric detonators. U.S. Pat. No. 6,704,657 describes the impedance monitoring of the detonation voltage and current, with and without a small signal rf injection in electric seismic detonators. In electronic detonators, the firing energy is usually stored onboard inside the electronic detonator and the current surge cannot be easily detected as a signal for the time break. Furthermore, the issuance of the FIRE command in electronic detonator does not always result in detonation all the time, sometimes due to damage to the detonator or the wire during loading. Thus conventional time break confirmation approaches may cause a time break to be signaled to a data acquisition system, but the detonator will not actually deploy, thereby increasing the cost of seismic exploration through acquisition and storage of useless data. Thus, it is desirable to provide more reliable techniques for detection and signaling of electric and electronic detonator time break values.
Seismic exploration using seismic sources and transducers rely upon accurate mapping and knowledge of seismic source location as well as the location of individual geophones. Shotpoints are used in seismic exploration to denote the grid location record of a seismic charge containing the detonator placed specifically in the array to be explored. This may contain the geographical records and other data. The shotpoint may be an 8-digit number e.g., 60531975 or 60611975, etc., which can be associated with a particular detonator based on the location in an exploration map at which the detonator and corresponding booster charge are to be placed, and are usually stored in a data acquisition system or other external media. Consequently, once a seismic detonator is successfully fired, further steps are needed to relay the corresponding fired-detonator shotpoint back to a control station (e.g., a remote recording facility sometimes referred to as a “doghouse”), typically by manual radio communication and manual entry of the detonator ID or shotpoint. Thus, it is desirable to improve seismic exploration processes and systems to facilitate timely provision of detonator shotpoint numbers and confirmation time break values for confirmed detonations.